The present invention relates to implantable cardiac stimulation devices, such as pacemakers, defibrillators and cardiaverters. More particularly, the present invention relates to an enhanced cardiac signal sensing system for sensing the occurrence of an R-wave.
The field of implantable cardiac devices, such as pacemakers, defibrillators, and cardioverters, is well known. These devices typically monitor cardiac response under a variety of conditions. Sudden cardiac death presently claims an estimated 400,000 lives annually in the United States. To prevent sudden death, rapid treatment of cardiac conditions is required. Rapid treatment can be provided by implantable cardiac devices only if the heart rate can be accurately and reliably sensed. One potentially catastrophic cardiac event is fibrillation, wherein the heart ceases to function as a blood pump. Unless a functional heart rate is quickly reestablished, death can occur within minutes. Once the abnormal heart rate is sensed, a normal heart rate or sinus rhythm may be reestablished by application of a large electrical shock on the order of 10 joules or more, to defibrillate the heart.
Another serious condition is tachycardia or tachyrhythmia, which is a rapid heart rate which may eventually lead to fibrillation. Once the rapid heart rate is sensed, a normal heart rate or sinus rhythm may be reestablished by application of a slightly more rapid rate of very low level pacing shocks of less than 100 micro joules to capture the heart rate and slow the heart to a normal rate. The normal sinus rhythm may also be reestablished by application of electrical shocks on the order of 1 joule to perform cardioversion on the heart.
The necessity of identifying persons likely to suffer tachyrhythmia or fibrillation has led to the preventative step of implanting a device known as an Implanted Cardioverter Defibrillation (ICD) device. The ICD device is electrically attached to the patient""s heart. Rather than relying on the patient or on attending medical personnel to identify a cardiac fibrillation event, the ICD device uses automatic triggering. With automatic triggering, accurate monitoring and detection of the heart rate is required to detect the cardiac fibrillation event and trigger the ICD device to defibrillate the heart.
Monitoring and detection of cardiac function typically involves electrical sensing of muscle and nerve cell depolarizations which can be correlated with cardiac muscle contractions. Electrodes are implanted in the heart which sense an electrical voltage which is measured over time to produce an electrocardiogram wave form. The electrocardiogram wave form under normal conditions includes a P wave, followed by a complex three-part wave form called the QRS complex, and then a T wave. Of these various components, the QRS complex or R-wave has a dominant amplitude feature and is therefore most typically used to sense the heart rate. The R-wave is the portion of the electrocardiogram wave form having the steepest slopes and the sharpest peaks. The heart rate is the interval between R-waves, and is sometimes termed the Rxe2x80x94R interval. R-waves typically have a peak amplitude in the range of about 5 to 15 mv during a normal sinus rhythm. T waves typically have a peak amplitude of about half of the R-wave amplitude. Noise and extraneous muscle movements typically have peak amplitudes in the range of about 0.1-1 mv. During fibrillation, the R-wave amplitude may diminish to as little as 20% of normal amplitude (e.g., 1.0 to 3.0 mv), thus making the R-wave amplitude indistinguishable from noise levels.
In one approach, detection systems sense the occurrence of R-wave electrical events and signals exceeding a preset constant voltage, where the constant voltage is fixed at a preset amplitude between 3.4 to 10 mv. Such triggering levels start at approximately 67% of the amplitude of a normal R-wave, which is higher than typical noise or T-wave amplitudes. During fibrillation, the R-wave amplitudes decrease to a range of 0.5 to 2.0 mv. Unfortunately, this type of prior art detection system having the preset and fixed amplitude detection threshold is incapable of distinguishing or sensing diminished or degenerating R-wave events during fibrillation. With this approach, R-wave information on heart rate is completely unreliable during fibrillation.
More recently, detection systems have attempted to address the problem of sensing diminishing R-wave amplitudes during fibrillation by employing a sensitivity threshold which starts at a preset amplitude and then subsequently becomes more sensitive until a floor threshold is reached. The preset amplitude is typically 67% of the preceding R-wave amplitude, and the floor threshold is typically set at 0.3 to 0.5 mv. This floor threshold prevents the sensitivity threshold from dropping to such low levels that the increased sensitivity begins to incorrectly detect noise as R-waves. This reduction in threshold amplitude and increase in sensitivity occurs in the form of an exponential decay with a time constant on the order of 1 to 1.5 seconds. This time constant is reset after each R-wave event. In one example, the xe2x80x9cSENTINELxe2x80x9d(trademark) brand ICD device, developed by Angeion Corporation, employs a detection mechanism in which the initial threshold is a preset percentage of the most recent R-wave peak amplitude, and the decay is a standard exponential. The threshold used in the xe2x80x9cSENTINELxe2x80x9d(trademark) device is lowered from the initial level until a constant floor threshold is reached. In another example, an initial threshold reset may be used which decays with a reverse exponential time constant to the floor threshold, where the floor threshold is set to a constant level which is greater than the noise level.
One problem with these approaches is temporary supersensitivity when a low amplitude R-wave occurs during normal sinus rhythm or tachycardia. This problem results from the decay being based upon a fixed time constant, or from the detection mechanisms resetting the initial threshold after each R-wave detection. Once the detection mechanisms observe a low amplitude R-wave, for example, of less than 3 mv, the initial sensitivity threshold is set to a low level of about 2 mv or less, and then proceeds to decline to the floor level of about 0.3 to 0.5 mv. At this level, the sensitivity threshold may allow noise to be falsely detected as R-waves. This problem is particularly severe in situations where the floor threshold is quite sensitive (e.g., when the floor setting is 0.4 mv and the noise level is 0.5 or 0.6 mv). This incorrect or false detection of noise as R-waves resets the initial threshold sensitivity to inappropriately low levels resulting in possible adverse effects to the patient. These adverse effects may include the initiation of electrical shock therapy to the heart based upon noise timing rather than R-wave timing. The electrical shock therapy would continue until a true R-wave was detected and the system could correct itself.
More recently, in another approach, an ICD detection method for sensing the occurrence of an R-wave attempts to distinguish R-waves from noise through the use of variable declining sensitivity thresholds. This approach, disclosed in U.S. Pat. No. 5,709,215 and developed by Angeion Corporation, considers the amplitude of at least the previous most recent R-wave, and determines a declining threshold of sensitivity which is used to recognize a subsequent electrical signal as an R-wave. With this approach, the amplitude of the previous R-wave may be classified, based on amplitude. Based upon the classification, a desirable time constant for the declining threshold of sensitivity is provided as either an exponential or a reverse exponential decay. Alternatively, piece wise use of various decay formulas may be combined and used.
This approach, while an improvement over previous approaches, is still dependent solely upon R-wave amplitude, and requires the setting of an appropriate sensitivity threshold to recognize the occurrence of an R-wave. Furthermore, this approach is still dependent upon distinguishing R-wave signal amplitudes from noise level amplitudes, and thus requires the R-wave to have a greater amplitude than the noise level amplitude, in order to avoid false recognition of noise as R-waves.
An R-wave detection method and apparatus for sensing the occurrence of an R-wave by utilizing characteristics of the R-wave which distinguish the R-wave from other portions of the electrocardiogram waveform is disclosed. With this approach, a first and second derivative is taken from an electrical signal measured from the heart. The electrical signal, known as the QRS complex or R-wave, is unique from other portions of the electrocardiogram waveform in that the product of the maximum value of the amplitude of the R-wave, the maximum value of the first derivative or slope of the R-wave, and the maximum value of the second derivative or slope transition of the R-wave, is greater for the R-wave than for any other portion of the electrocardiogram waveform. Once the values of the peak of the R-wave, the peak of the first derivative of the R-wave, and the peak of the second derivative of the R-wave are determined, the three peak values are multiplied together to provide an output product value. The output product value is greater in amplitude than either the value of the peak of the R-wave, the value of the peak of the first derivative of the R-wave, or the value of the peak of the second derivative of the R-wave , and is also higher than any other portion of the electrocardiogram waveform including background noise.
In a preferred embodiment of the present invention, an R-wave detector is provided to measure the heart rate from a tip-ring signal provided from sensing electrodes implanted in the heart. The tip-ring signal is an electrical voltage signal which is provided to a buffer which may optionally perform signal preconditioning. This signal preconditioning may include filtering or automatic gain control to bring the signal amplitude up to or down to any desired level. The output of the buffer is provided to a first self-clearing peak detector which provides an output proportional to the peak of the electrical signal, which is also the zeroth derivative of the electrical signal. The output of the buffer is also provided to two cascade differentiators which determine the first and second derivatives of the electrical signal. The output of the first differentiator is provided to a second self-clearing peak detector which provides an output proportional to the peak of the first derivative of the electrical input signal. The output of the second differentiator is provided to a third self-clearing peak detector which provides an output proportional to the peak of the second derivative of the electrical signal. The peak detectors are self-clearing as they maintain the output for a predetermined time before clearing. The outputs of the first, second and third self-clearing peak detectors are provided to a multiplier which multiplies the outputs together to provide a product output. The multiplier product output is proportional to the product of the peaks of the zeroth, first and second derivative of the electrical input wave form. The multiplier product output is provided to a self-clearing fraction of peak detector which provides an output response once the product output from the multiplier is received. The self-clearing fraction of peak detector may optionally couple to a one-shot device which provides a digital pulse output having Complimentary Metal Oxide Semiconductor (CMOS) voltage levels.
In an alternative embodiment of the present invention, self-clearing peak to peak detectors are used rather than peak detectors to provide a greater amplitude output to the multiplier. In the alternative embodiment, the outputs of the buffer, first differentiator and second differentiator provide the zeroth, first and second derivatives respectively of the input electrical signal wave form to the first self-clearing peak to peak detector, the second self-clearing peak to peak detector and the third self-clearing peak to peak detector. The zeroth, first and second derivative have both positive and negative peaks which are inherent in the R-wave signal, and which may be measured by the self-clearing peak to peak detector. The self-clearing peak to peak detectors provide an output proportional to the sum of the peak positive amplitude and the inverted peak negative amplitude of the input signal. Since this sum is greater than either the positive or negative peak alone, a higher level output response from the self-clearing peak to peak detector is provided to the multiplier, thus improving the ability to detect low amplitude electrical signals measured from the heart.
In another alternative embodiment of the present invention, a buffer is provided to receive the electrical input signal measured from the heart, and perform any desired signal conditioning before coupling the electrical input signal to a self-clearing peak to peak detector and an input of a comparator. The self-clearing peak to peak detector output couples to a resistor divider network, and an output of the resistor divider network couples to the other input of the comparator. The comparator compares the output of the resistor divider network, which is a fraction of the peak to peak value of the electrical input signal, to the electrical input signal, and provides an output when the R-wave is detected. Since the self-clearing peak to peak detector provides an output proportional to the sum of the peak positive amplitude and the inverted peak negative amplitude of the detected R-wave, which is greater than the positive or negative peak of the R-wave signal provided from the buffer, the R-wave can be easily detected by the comparator. The output of the comparator may optionally couple to a one-shot device which provides a digital pulse output having CMOS voltage levels.